Device for stimulating bone growth, especially for the osteosynthesis of bone fragments and/or for fixing bone fractures

ABSTRACT

Apparatus for promoting bone growth, especially for osteosynthesis of bone fragments and/or fixation of bone fractures is described. This apparatus comprises at least one piezoelectric element, which is associated with an implant or like bone fixation means and which, under the action of forces, generates electrical pulses which serve as a stimulant for bone growth. The at least one piezoelectric element is an integral component of the implant.

The invention relates to an apparatus for promoting bone growth,especially for osteosynthesis of bone fragments and/or fixation of bonefractures, according to the preamble of claim 1.

The concern in the present case is to promote bone growth, especially inthe field of bone fractures, but also for the purpose of reducingosteoporosis, which is increasingly becoming an economic requirement. Inaustria alone, for example, 150 million euros per year are having to bespent on caring for femoral neck fractures, with the secondary costs nothaving been included in that figure. On average, every third woman from60 to 70 years of age suffers from osteoporosis and, among theover-80's, it is even ⅔ of all women that are affected.Osteoporosis-related fractures result in immobility and a need to becared for, pain and loss of quality of life. The mortality rate duringthe rehabilitation phase is high. The medical costs for treatingosteoporosis-related fractures to be met annually in the usa and europeare currently about 25 billion euros. That figure does not include theindirect secondary costs such as the costs for rehabilitation and care,for sick-leave, loss of work and long-term institutional care.

There is accordingly a huge need for a remedy and for reducing theafore-mentioned costs.

From de 4 102 462 a1, which was originated by the inventor, there isknown a purely mechanical apparatus for promoting bone growth. Theusually elongate stabilising element described therein for theosteosynthesis of bone fragments has, despite its thin-walledconstruction, a high degree of rigidity, that being brought about by across-section of arcuate, wavy, meandering, zigzag or like shape. It hasbeen shown in practice that the said stabilising element is alsowell-tolerated and, in addition, simple to implement. The saidstabilising element has been found to be especially suitable formechanical support and/or assistance in the healing of complex bonefractures. As a result of the fact that the known stabilising elementhas only linear contact with the associated bone and is made frombiologically tolerable material such as, for example, titanium ortitanium alloy, which is preferably roughened on the surface, bonegrowth is promoted in positive manner.

Alternative investigations have shown that bone growth can be furtherpromoted by electrical stimulation. For that purpose, two differentmethods have been used hitherto:

The electrical stimulation can be carried out, on the one hand, directlyby means of conductive coupling by way of supply wires or, on the otherhand, inductively by way of an external electromagnetic field.

Direct (conductive) stimulation has the disadvantage that a sequence ofelectrical pulses is produced transcutaneously by way of supply wiresfrom outside the body passing through the skin of a patient. Once thebone has healed, the supply wires also have to be operatively removed.Inductive stimulation requires a considerable outlay on externalapparatus for the generation of electromagnetic fields.

In practical use, both of the afore-mentioned methods for electricallystimulating bone growth have a further substantial disadvantage: becauseof the external devices, such as an electric pulse source for thegeneration of electrical stimulation pulses, supply wires and likedevices, both methods can be carried out only under supervision by, forexample, a medical practice or hospital. As a result, use is limited toparticular times. For especially rapid healing, however, application ofthe methods without limitation in terms of time and on demand would beadvantageous.

Finally, both the afore-mentioned methods do not take into account thestate of motion of a patient. The methods are in no way adaptive and areapplied to the patient without regard to the patient's absorptioncapacity—also referred to medically as “resonance capacity”. In thatcontext, for example, adaptation of the intensity and frequency ofelectrical pulses in dependence upon the loading on the bone to behealed would be desirable for stimulation of bone growth.

The afore-mentioned disadvantages can be avoided by means of anapparatus comprising at least one piezoelectric element associated withan implant or directly with the bone, as is described in U.S. Pat. No.6,143,035 or the corresponding ep 1 023 872 a2. That proposal avoidsexternal apparatus and/or supply wires from an external electrical pulsesource. In addition, the known proposal has the advantage that theelectrical pulses generated by the piezoelectric element for thestimulation of bone growth are adaptive, that is to say the stimulationis matched to the actual state of motion and loading of a patient.

In the case of the proposal according to U.S. Pat. No. 1,143,035 it isdisadvantageous that the piezoelectric element(s) is/are mounted on theoutside of an implant, for example a femoral stem. The piezoelectricelement(s) accordingly project(s) outwards from the implant. As aresult, the implant loses the original accuracy of fit, with theconsequence that there is a risk of its becoming loose. In addition,there is a risk that, as a result of external loading, the piezoelectricelement(s) will become separated from the implant and thereforeineffective. When a piezoelectric element is mounted on the outside ofan implant it is no longer possible to adhere to the requisite maximumtolerance of from 0.1 to 0.25 mm over the entire surface of the implantin relation to a previously reamed-out space for accommodating theimplant.

The problem underlying the present invention is accordingly to providean apparatus of the kind mentioned at the beginning whereinpiezoelectric elements do not project out beyond the implant surface sothat the implant can be implanted in customary manner. In addition, itshould be ensured that the forces acting on the implant act directly onthe piezoelectric element associated with the implant.

In accordance with the invention the problem is solved by means of thefact that at least one piezoelectric element is an integral component ofthe implant. The implant preferably consists at least in part of apiezoelectric ceramic.

The fact that the piezoelectric element is an integral component of theimplant should ensure that the external contour of the implant remainsunchanged. Consequently, the implant can be implanted in customarymanner. As a result of the embedding of the piezoelectric element withinthe implant it is also ensured that external forces act directly andenduringly on the piezoelectric element by way of the implant. Theimplant always defines one electrical pole of the piezoelectric element,the second pole being defined by a contact element coming into contactonly with the surrounding bone and made from an electrically conductive,especially metallic, material tolerable to humans. That contact elementpreferably consists likewise of implant material. It may be of strip-,disc- or button-like construction, that being dependent, in the lastanalysis, on the geometry of the opening of the accommodating pocket forthe piezoelectric element.

In order to ensure that the implant retains its original contour inspite of the integrated piezoelectric element, the piezoelectric elementis preferably so arranged inside an implant pocket which is open towardsthe bone that it terminates substantially flush with the surface of theimplant.

Specific embodiments of implants having associated piezoelectricelements are described in claims 5 ff. They are also explainedhereinbelow in greater detail with reference to the accompanyingdrawings, in which

FIG. 1 shows, partly in longitudinal section and partly in side view, atooth implant having a piezoelectric element;

FIG. 2 shows the tooth implant of FIG. 1 in cross-section along lineii-ii in FIG. 1;

FIG. 3 shows a bone-inducing femoral neck pin, partly in side view andpartly in longitudinal section, showing the implantation within afemoral neck;

FIG. 4 shows, in section, a hip-joint socket together with abone-inducing piezoelectric element;

FIG. 5 shows, in side view, a femoral stem having a piezoelectricelement on the anterior/posterior face, on the one hand, and a furtherpiezoelectric element laterally;

FIGS. 6 and 7 show the femoral stem of FIG. 5 in cross-section alongline vi-vi and along line vii-vii in FIG. 5;

FIG. 8 shows, in section, a femoral sled showing fixation screws havinga piezoelectric element;

FIG. 9 shows, in section, a tibial component, the bone fixation screwsof which are each provided with a piezoelectric element; and

FIG. 10 is a cross-section through a semi-circular rod element forstabilising a bone fracture, the hollow space in which, facing the bone,is filled by a piezoelectric element, in cross-section.

FIGS. 1 and 2 show, in partial longitudinal section and cross-section, atooth implant 10 for an artificial tooth 11. The part which isanchorable in the (jaw-)bone 12 is in the form of a bone screw 13 havingan external thread 14, the upper part of the bone screw 13 having a cone15, on which the artificial tooth 11 can be placed. The threaded part ofthe bone screw 13 is of hollow construction and is provided with twolongitudinal slots 16 arranged diametrically opposite one another. Thelongitudinal hollow space 17 of the threaded portion of the bone screw13 is filled with piezoelectric ceramic, which defines the piezoelectricelement according to the invention. In the region of the twolongitudinal slots 16 there extend electrically conductive contactstrips 19, which are made preferably from the same material as the bonescrew 13, namely titanium or a titanium alloy, that is to say a materialwhich is tolerable to humans. The contact strips 19 are in contact onlywith the bone 12, on the one hand, and with the piezoelectric ceramic18, on the other hand, that is to say not with the implant screw 13. Thecontact strips 19 accordingly define the opposite pole to the bone screw13. The latter preferably forms the negative pole whereas the contactstrips 19 define the positive pole.

The longitudinal slots 16 are substantially filled by the contact strips19 so that the original external contour of the bone screw 13 isvirtually unchanged. The described tooth implant is especially highlyeffective, more specifically because of the dynamic loading of thepiezoelectric element 18 during chewing. The electrical signals orpulses produced in the process bring about more rapid healing of thejaw-bone 12.

From previously obtained findings in the electrical stimulation of bonehealing it is known that an effective current intensity (direct,alternating or square-wave pulse current) of about 10-100 μa is best forpromoting the bone growth. The piezoelectric element is thereforepreferably so constructed that, on normal loading of the bone structure,a current having an effective current intensity of about 10-100 μa isgenerated.

The piezoelectric element 18 preferably consists of a piezoelectricceramic. In this context, zirconate or titanate ceramics have been foundto be especially suitable for the area of surgical/orthopaedicapplications, because they are tolerable to the body and can be readilyintegrated into the body. Other piezoelectrically active ceramics whichare tolerable to the body, such as quartz ceramic, are also feasible.

FIG. 3 shows, partly in longitudinal section and partly in a side view,the implantation of a femoral neck pin 21 having a piezoelectric element20 in the region of a femoral neck which is at risk of fracture.Reference numeral 22 denotes the femoral neck subject to the risk inquestion. The femoral neck pin 21, which is made from titanium or atitanium alloy, is of similar hollow construction to the bone screw 13.The hollow space is filled with piezoelectric ceramic, which defines thepiezoelectric element 20. In this case too, there are provided twolongitudinal slots 23 arranged diametrically opposite one another, inthe region of which there are located electrically conductive contactstrips 24 corresponding to the contact strips 19 according to FIGS. 1and 2, more specifically in such a manner that they are in contact withthe ceramic 20, on the one hand, and with the surrounding bone, on theother hand. The contact strips 24 therefore form the opposite pole tothe pin 21. It should be mentioned at this point that the bone structurehas a crystalline structure and reacts on mechanical loading withpiezoelectric pulses. In the converse case, the bone reacts with amechanical moment, which again results in bone formation. Thatreciprocal action of mechanical moments and piezoelectric pulses isutilised in accordance with the invention.

The femoral neck pin shown in FIG. 3 is used in this instance forprophylaxis but can be used equally well for healing a femoral headfracture.

In very similar manner, bone or pedicle screws can be introduced atother sites in the bone for the purpose of prophylaxis. Bone or pediclescrews of such a kind have a threaded part corresponding to that of thebone screw 13 in FIGS. 1 and 2.

FIG. 2 shows a hip-joint socket or hip socket 25, which is screwed intothe hip bone 26. Reference numeral 27 in FIG. 4 denotes thecorresponding screw thread. In the case of old and very old patients,the bone recedes in the region of a cementless hip socket implant, suchas the hip socket 25 shown here. The hip socket is then held in positiononly by thin bone trabeculae. In order to prevent that problem, the hipsocket 25 shown is provided with openings 28 in its bottom, which areeach filled with piezoelectric ceramic 29. The piezoelectric ceramicalso extends over the entire inside of the bottom of the socket and issubjected to pressure by the inlay (not shown in FIG. 4). On the outsideof the socket, the piezoelectric ceramic 29 is in contact with the bone26 by way of push-button-like contact elements 30. The push-button-likecontact elements 30 are in contact with the piezoelectric ceramic 29, onthe one hand, and with the surrounding bone 26, on the other hand;otherwise, they are isolated from the socket 25. The contact elements 30accordingly form the electrically opposite pole to the socket 25. In thecementless hip socket implant shown, bone growth is promoted, on the onehand, by the tips of the thread 27 and, on the other hand, by thepiezoelectric system shown. Especially as a result of the latter, boneformation takes place in the direction of the implant, which becomesincreasingly stabilised in the course of time. As a result of themeasures described, therefore, exactly the opposite effect occurs tothat which would normally be expected, namely bone formation instead ofbone loss.

FIGS. 5 to 6 show a femoral stem having a pocket 31 and 32 formed in ananterior and a lateral position for accommodating a piezoelectricceramic or piezoelectric element 33 and 34, respectively. Theaccommodating pockets 31 and 32 are each in the form of longitudinalgrooves and each is of approximately semi-circular cross-section.Contact strips 35 and 36 are embedded in the piezoelectric ceramic 33and 34, respectively, on the side facing the bone, more specifically insuch a manner that the piezoelectric ceramic including the contact stripterminates flush with the external surface of the implant, in this casethe femoral stem 37.

The femoral stem 37 can also be provided with elements 33, 34corresponding to piezoelectric elements on the posterior and/or themedial face, that being dependent, in the final analysis, on thestructure of the patient's bone. In this instance too, the contactstrips 35, 36 again form the positive electrical pole of thepiezoelectric element 33 and 34, respectively, whereas the implantitself, namely the femoral stem 37, defines the negative pole.

The examples described also show very clearly that no wires areinstalled for the transmission of current pulses. The implants areintended to have substantially their original shape so that they can beimplanted in customary manner.

FIGS. 8 and 9 show, in diagrammatic longitudinal section, a femoral sled38 on the one hand and a tibial plateau 39 on the other hand, a bearingbody 40 made of polyethylene or like plastics material which istolerable to humans being mounted fixedly or displaceably (translationand/or rotation) on the latter. Both the femoral sled and also thetibial plateau are fixed to the femur 41 and the tibia 42, respectively,by means of bone screws 43 and 44. The bone screws 43, 44 have athreaded part, which corresponds to that of the bone screw 13 accordingto FIG. 1. In the case of the bone screws 43, 44 too, a longitudinalhollow space is provided, which is filled with a piezoelectric ceramic,forming in each case a piezoelectric element 45, 46. In the region ofthe longitudinal slots there are again provided contact strips 47, 48,which are in contact with the piezoelectric ceramic on the one hand andwith the surrounding bone on the other hand.

The piezoelectric element 45 and 46 is, in each case, somewhat conical,more specifically widening out conically towards the end of the screw sothat the threaded part of the bone screws 43, 44 is correspondinglyexpanded outwards in a radial direction, thereby achieving a better holdin cancellous bone.

FIG. 10 shows, in cross-section, an elongate stabilising element inaccordance with de 4 102 462 a1, more specifically in association with abone 50. Reference numeral 49 denotes the stabilising element, which isin the form of an elongate half-tube. It has only linear contact withthe bone surface, that linear contact being interrupted by tips 51, 52spaced longitudinally apart from one another, which penetrate into thebone. The stabilising element shown is held in position by means of aholding band 53 wrapped around the bone and the stabilising element 49.FIG. 10 shows only part of the holding band 53. Above all, the closingelement for the two free ends of the holding band is not shown. To thatextent, however, it represents known prior art, also originated by theinventor.

The hollow space between the stabilising element 49 and the surface ofthe bone is filled with a piezoelectric ceramic. On the side facing thesurface of the bone, an electrically conductive contact strip 55 isembedded in the ceramic 54. The contact strip is isolated from thestabilising element 54 by the ceramic as in the previously describedembodiments and defines the opposite pole to the stabilising element 49,which is made from titanium or a titanium alloy. In this instance too,the piezoelectric element 54 is an integral part of the stabilisingelement 49.

When more than one piezoelectric element is provided, at least twopiezoelectric elements can be electrically connected in series in orderto obtain a higher electrical voltage. Alternatively, the piezoelectricelements can also be electrically connected in parallel, as a result ofwhich a higher current intensity can be obtained, it being fundamentalthat the effective current intensity of 10-100 μa is achieved. Thepiezoelectric elements should then be connected in the correspondingmanner.

As already mentioned, the piezoelectric elements can be associated witha very great diversity of implants including, for example, an artificialpatella. In that respect there are no limits.

Loading of the piezoelectric element is effected by way of the implant,on the one hand, and the bone, on the other hand, it also being possiblefor pressure to be exerted by the musculature.

All features described in the application documents are claimed to be ofinventive significance insofar as they are, on their own or incombination, novel with respect to the prior art.

List of Reference Numerals

10 tooth implant

11 artificial tooth

12 (jaw-)bone

13 bone screw

14 external thread

15 insertion cone

16 longitudinal slot

17 longitudinal hollow space

18 piezoelectric element (ceramic)

19 contact strip

20 piezoelectric element (ceramic)

21 femoral neck pin

22 femoral neck

23 longitudinal slot

24 contact strip

25 hip socket

26 hip bone

27 screw thread

28 opening in bottom

29 piezoelectric element (ceramic)

30 contact element

31 pocket

32 pocket

33 piezoelectric element (ceramic)

34 piezoelectric element (ceramic)

35 contact strip

36 contact strip

37 femoral stem

38 femoral sled

39 tibial plateau

40 bearing body

41 femur

42 tibia

43 bone screw

44 bone screw

45 piezoelectric element (ceramic)

46 piezoelectric element (ceramic)

47 contact strip

48 contact strip

49 stabilising element

50 bone

51 tip

52 tip

53 holding band

54 piezoelectric element (ceramic)

55 contact strip

1-11. (canceled)
 12. An apparatus for promoting growth selected from thegroup consisting of bone growth, osteosynthesis of bone fragments,fixation of bone fractures, and osteosynthesis of bone fragments withfixation of bone fractures, said apparatus comprising: at least oneimplant; at least one piezoelectric element associated with saidimplant, said piezoelectric element under the action of forces,generates electrical pulses which serve as a stimulant for bone growth,said at least one piezoelectric element forming an integral component ofsaid at least one implant; and at least one contact element operative tocome into contact only with surrounding bone and said piezoelectricelement, said at least one contact element being made from electricallyconductive material tolerable to humans; and wherein said implantdefining one pole and said contact element defining the other pole ofsaid piezoelectric element; and said piezoelectric element beingarranged within said implant or within an implant pocket open towardsthe bone.
 13. The apparatus of claim 12, wherein said piezoelectricelement terminates substantially flush with the surface of said implant.14. The apparatus of claim 12, wherein said implant is in the form of adowel defining a central hollow space, said piezoelectric element beinglocated in said central hollow space of said implant.
 15. The apparatusof claim 12, wherein said implant is selected from the group consistingof a pin-like holder for an artificial tooth, a bone or pedicle screw, abone fixation pin, and a bone fixation element.
 16. The apparatus ofclaim 12, wherein said implant is a hip-joint socket defining at leastone opening in the bottom thereof, said piezoelectric element beingarranged to be located in said opening in the bottom of said hip-jointsocket.
 17. The apparatus of claim 16, wherein said piezoelectricelement is arranged in and fills said opening in the bottom of saidhip-joint socket and is integrally connected to a piezoelectric layerextending over at least part of the inside of the bottom of saidhip-joint socket.
 18. The apparatus of claim 12, wherein saidpiezoelectric element is so constructed that, on normal loading of thebone structure, a current having an effective current intensity of about10-100 μA is generated.
 19. The apparatus of claim 12, wherein saidpiezoelectric element is made from a material selected from the groupconsisting of a piezoelectric ceramic, a zirconate ceramic and atitanate ceramic.
 20. The apparatus of claim 12, wherein at least two ofsaid piezoelectric elements are provided and have an electricalconnection selected from the group consisting of series electricalconnection, and parallel electrical connection.
 21. The apparatus ofclaim 12, wherein said at least one implant is made of metallicmaterial.
 22. The apparatus of claim 12, wherein said implant defines anegative pole and said contact element defines a positive pole.